A wet chemical approach is selected over dry plasma cleaning because it provides highly selective, damage-free removal of contaminants without introducing ionic bombardment or altering the subsurface mechanical properties of the oxide .
In depth
The Light Shield (LS) or Aperture Grid is a critical structu
ral component in back-illuminated complementary metal-oxide-semiconductor (BSI CMOS) image sensors, designed to suppress optical crosstalk and precisely define the pixel's optical aperture . In the LS_GRID module, after the sequential deposition of the barrier, the main light-blocking metal (typically W), and the Oxide Grid Seal Layer, the wafer must undergo a Pre Litho Cleaning step prior to photoresist application . This specific step is fundamentally distinct from earlier front-end pre-litho cleans (e.g. (Engineering Practice), steps #7 or #13) because it acts on a newly deposited oxide seal layer covering a highly stressed metallic grid stack, rather than acting on planar silicon or epitaxial substrates . Ensuring a pristine, defect-free oxide surface at this stage is essential to guarantee perfect photoresist adhesion and to prevent sub-wavelength lithographic defects during the subsequent critical Light Shield/Aperture Grid Photo step . The core physical and chemical mechanism of this step relies on wet-chemical oxidation and dissolution to remove organic residues and particulate contamination . Solutions such as the ammonia hydrogen peroxide mixture (APM) are typically employed to achieve this surface preparation . In this system, hydrogen peroxide acts as a strong oxidant that breaks down hydrocarbon bonds in organic contaminants into volatile or soluble byproducts, mimicking the oxidative degradation mechanisms utilized in post-etch cleaning processes . Simultaneously, the alkaline nature of the ammonia slightly etches the superficial molecular layers of the oxide seal layer, undermining adhered particles . Furthermore, the high pH environment induces a strong negative zeta potential on both the oxide surface and the detached particles, establishing a mutual electrostatic repulsion that effectively prevents particle redeposition . A wet chemical approach is selected over dry plasma cleaning because it provides highly selective, damage-free removal of contaminants without introducing ionic bombardment or altering the subsurface mechanical properties of the oxide . The concentration ratios and temperature of the cleaning solution must be tightly controlled; an excessive oxidant concentration drives rapid oxidation but may lead to surface roughening if the corresponding dissolution rate is not balanced . Conversely, excessive alkalinity increases the oxide removal rate, risking the consumption of the thin Oxide Grid Seal Layer and potentially exposing the underlying metallic grid to subsequent corrosive processing environments (Engineering Practice). Therefore, a highly dilute, mild wet chemistry is preferred to achieve optimal particle removal while maintaining strict film thickness preservation . At the 40nm technology node, the dimensional tolerance for the aperture grid is exceptionally tight, directly impacting the light-gathering efficiency, quantum yield, and signal-to-noise ratio of the photodiode array . Any nanoparticle or molecular organic contamination present before lithography can cause local defocusing or micro-masking during the deep ultraviolet (DUV) exposure (Engineering Practice). This optical perturbation would directly translate into a distorted metal grid after the W Etch step, degrading the pixel's optical isolation . Consequently, this pre-litho clean must achieve near-zero defectivity at the nanoscale without introducing any macroscopic surface roughness that could compromise the anti-reflective properties or structural integrity of the BSI optical stack .
Risks & Challenges
[High] Particle Redeposition and Micro-masking: If the zeta potential is not properly maintained due to an incorrect solution pH, electrostatically repelled particles can reattach to the oxide surface . During lithography, these residual particles act as micro-masks or cause localized focal deviations, leading to severe pattern distortion in the subsequent metal grid etch .
[Medium] Oxide Seal Layer Over-etching: Prolonged exposure to alkaline wet chemistry can cause excessive isotropic dissolution of the Oxide Grid Seal Layer . If the seal layer is compromised, the underlying metal grid may be exposed to subsequent processing chemicals, leading to galvanic corrosion or material loss (Engineering Practice).
[Low] Surface Roughening: Imbalanced oxidation and dissolution rates in the wet chemical mixture can induce local nanometer-scale roughening on the oxide surface . This localized roughness can alter the surface contact angle and adhesion properties, potentially causing photoresist peeling during development or degrading the geometric uniformity of the aperture grid (Engineering Practice).